A circle has a circumference of $6\pi$. It has an arc of length $\dfrac{21}{4}\pi$. What is the central angle of the arc, in radians? ${6\pi}$ ${\dfrac{7}{4}\pi}$ $\color{#DF0030}{\dfrac{21}{4}\pi}$
Explanation: The ratio between the arc's central angle $\theta$ and $2 \pi$ radians is equal to the the ratio between the arc length $s$ and the circle's circumference $c$ $\dfrac{\theta}{2 \pi} = \dfrac{s}{c}$ $\dfrac{\theta}{2 \pi} = \dfrac{21}{4}\pi \div 6\pi$ $\dfrac{\theta}{2 \pi} = \dfrac{7}{8}$ $\theta = \dfrac{7}{8} \times 2 \pi$ $\theta = \dfrac{7}{4}\pi$ radians